Tutorial 16c: Accurate Clocks

Perfectly synchronized clocks can measure the bits
anywhere in the middle.

Clock Accuracy
So just how accurate does a clock need to be for UART? First, consider a perfect pair of clocks, each operating at exactly the same frequency. In this case, it doesn't much matter where we measure the incoming bits, as long as we measure after the bit has changed to the next one. If there's some error, however, a slow clock will measure later and later, until it missed a bit completely. A fast clock will likewise measure earlier and earlier, until it measures the same bit twice. Not knowing before hand if your clock is fast or slow, it makes sense to shoot for the middle of the bit; that way you have as much time as possible before the error takes over and you make a mistake.

Errors can add up very quickly when there are
small differences between clocks.

Looking at the plot on the left, using clocks that are off by only 6%, notice where the first problem occurs: bit 9 is completely skipped! Using these clocks, we can't even reliably send one frame of data! (By the way, this type of transmission error is classified as a frame error.) My choice of 6% for this plot isn't arbitrary; the error quoted in the datasheets for the MSP430 give the calibrated DCO frequencies (calibrated, mind you; not just the DCO in general) a tolerance over temperature and any other changes of ±3%. If we use calibrated DCO as the clock for both sender and receiver, we could feasibly have as much as a 6% error, making UART transmission completely unreliable!

In reality, not all hope is lost; this is a worst case scenario, and likely the clock in your computer (assuming you want to communicate with it instead of another MSP430) is more accurate than that. A total of 5% tolerance between the clocks would, on average, encounter a frame error on bit 11, and 4% tolerance on bit 13, so as long as we have better tolerance than that and have a short gap between frames to "resynchronize", we should be able to communicate. Note this does mean we are limited in the frame sizes we can use reliably, and our actual data transmission rate will be a little less since we need a recovery between frames. If you want fast communication, you need an accurate clock!

Crystal Oscillators
The best option for accuracy in a clock is to use a crystal. The watch crystal that comes with the LaunchPad is accurate over a wide temperature range to 20 ppm (0.002%)! There are some disadvantages, however: crystals take a while to stabilize, and use more power. In addition, at 32,768 Hz, we can't achieve very high transmission rates. Typically we want at least 16 clock cycles per bit to accurately catch the start bit and start sampling at the center of each bit. Using that rule of thumb, the highest bit rate we could attain with the watch crystal is 2048 baud. Larger MSP430 devices allow for high frequency crystals, but the G2xx series that are compatible with the LaunchPad do not. However, if we're careful, getting transmission rates up to 9600 baud with the watch crystal can be done.

Soldering the crystal to your LaunchPad might seem a daunting task with such a small part, but in reality it's not terribly difficult. A little bit of patience (mostly in the form of a little piece of masking tape) is all you need. Aside from a soldering iron and solder, of course. If you have not already put a crystal on your LaunchPad and would like to, there are a number of good demonstrations of the technique for soldering the crystal to the board available on YouTube. For now, I'll assume you've successfully put it on.

You may have noticed a couple of empty pads near the crystal for capacitors. For a crystal to oscillate at the right frequency, it needs to see a particular capacitance to ground. If the capacitance is off, the frequency may be off, or the crystal may not oscillate at all. The crystal included in the LaunchPad kit wants to see 12.5 pF. The MSP430 crystal inputs also have user-selectable capacitances internal to the device. The user can select from 1, 6, 10, and 12.5 pF as the effective capacitance seen by the crystal. (The device defaults to 6 pF.) The selection is done by the two XCAPx bits in the BCSCTL3 register.

If for some reason you need a capacitance other than one of these, you can solder the proper capacitors to the pads on the outside. Unfortunately, it's not as easy as putting 12.5 pF capacitors on the pads; the capacitors you put on will be in parallel with the capacitance from the traces and the chip itself. The formula for calculating the right load capacitors is: C1 = C2 = 2*C_Load - (Cp + Ci). C_Load would be whatever capacitance the crystal expects to see, Cp any parasitic capacitance from traces etc., and Ci the Capacitance of the MSP430 device. The last two terms can be assumed to be whatever is set in XCAPx. So for a crystal that wants 18 pF, you would want to put 30 pF capacitors on the board (using the default 6 pF).

Most of what's needed to use the watch crystal is set as the default values in the BCS+ module. To use the supplied crystal, you really only need two lines:

BCSCTL3 |= XCAP_3;       // 12.5 pF for LaunchPad crystal
__delay_cycles(55000);   // let crystal stabilize

The first line sets up the proper capacitance for your crystal, and should be changed if you're using a different capacitance value. The second line lets enough time pass for the crystal to stabilize at the operating frequency. How long do you really need to wait? Typically you need a few hundreds of milliseconds. The above code will wait for 55000 clock cycles; at the default DCO of 1.1 MHz, that's about 50 milliseconds. Depending on your application, you can wait longer if necessary.

Self-Calibrating the DCO
If you're paying attention, you might have spotted another problem with the watch crystal: how do we get a standard clock rate from the crystal? 9600 baud is 3.41 clock cycles. 1200 baud uses more clock cycles, so is more reliable, but it's still 27.31 clock cycles. The closest division we can get is 27 clock cycles, which would correspond to 1213.6 baud, an error of 1.1%. It seems we've lost a lot of the accuracy we gained using the crystal! There are crystals available that divide perfectly into the standard baud rates. A 7.3728 MHz crystal, for example, has exactly 768 clock cycles per bit for 9600 baud. If you use an MSP430 device that allows for a high frequency crystal, this is an excellent choice for serial communication at standard rates. Another option, however, is to use the DCO.

But wait, didn't we just argue that the calibrated values of the DCO are too large to be reliable? Yes, but under certain circumstances we can do better. For one, the error margins quoted in the data sheet cover both the entire temperature range where the MSP430 can be used-- from -40° to 85°F-- and the entire range of voltages at which it can operate-- from 1.8 to 3.6 V. Generally we won't be operating in such extremes. Even if we are, as long as the temperature and operating voltage aren't going to change too much during operation, we can recalibrate the DCO to that particular configuration! The accuracy of the clock will be much better than the quoted 3% in this case; the datasheet even specifies that from 0° to 85°F, the calibration is good to 0.5% at 3 V. (Obviously a consistent voltage is the real key to a stable DCO.)

Calibrating the DCO is quite simple, but it requires a very accurate clock to compare the DCO against; this is a job for a crystal. The idea is simple-- use the crystal to time an accurate interval (say 1 s.) We know how many oscillations should occur for a given frequency in that interval, so we adjust the DCO until we get as close to that as we can get. Then we save the values for DCOCTL and BCSCTL1, and we have an accurate calibration for our clock! Here's how all the magic happens:


void Set_DCO(unsigned int Delta) {    // Set DCO to selected
                                      // frequency
  unsigned int Compare, Oldcapture = 0;


  BCSCTL1 |= DIVA_3;                  // ACLK = LFXT1CLK/8
  TACCTL0 = CM_1 + CCIS_1 + CAP;      // CAP, ACLK
  TACTL = TASSEL_2 + MC_2 + TACLR;    // SMCLK, cont-mode, clear


  while (1) {
    while (!(CCIFG & TACCTL0));       // Wait until capture     
                                      // occured
    TACCTL0 &= ~CCIFG;                // Capture occured, clear 
                                      // flag
    Compare = TACCR0;                 // Get current captured 
                                      // SMCLK
    Compare = Compare - Oldcapture;   // SMCLK difference
    Oldcapture = TACCR0;              // Save current captured 
                                      // SMCLK


    if (Delta == Compare)
      break;                          // If equal, leave  
                                      // "while(1)"
    else if (Delta < Compare) {
      DCOCTL--;                       // DCO is too fast, slow 
                                      // it down
      if (DCOCTL == 0xFF)             // Did DCO roll under?
        if (BCSCTL1 & 0x0f)
          BCSCTL1--;                  // Select lower RSEL
    }
    else {
      DCOCTL++;                       // DCO is too slow, speed 
                                      // it up
      if (DCOCTL == 0x00)             // Did DCO roll over?
        if ((BCSCTL1 & 0x0f) != 0x0f)
          BCSCTL1++;                  // Sel higher RSEL
    }
  }
  TACCTL0 = 0;                        // Stop TACCR0
  TACTL = 0;                          // Stop Timer_A
  BCSCTL1 &= ~DIVA_3;                 // ACLK = LFXT1CLK
}

This code comes from the dco_flashcal.c example code available with most of the MSP430 devices. The example code file for the G2xx1 devices seems to not have it; I copied this from the F21x2 examples and changed TACCTL2 to TACCTL0 to be compatible with LaunchPad devices.

It's a bit of code to sort through, but it turns out to be straightforward. ACLK is configured to use the watch crystal divided by 8-- 4096 Hz. The timer is set to capture mode, triggering off a rising edge of CCI0B. (From the G2231 or G2211 datasheets, this corresponds to ACLK. If these terms seem confusing, review the tutorial on the capacitance meter which used the capture mode.) The timer itself is running off SMCLK, sourced by the DCO. If we want to calibrate 2 MHz, then in one clock cycle of ACLK, we expect 2 MHz/ 4096 Hz = 488.3 SMCLK cycles. We pass the value 488 to this routine, which starts the clocks and timers. When a capture occurs, it checks to see if more or fewer cycles of SMCLK have happened, and adjusts DCO and RSEL accordingly. It repeats this, until it finds the configuration that returns exactly 488 cycles in the interval. The values in DCO and RSEL are then the calibration values we want to save; we just look at the DCOCTL and BCSCTL1 registers and save their values for future use.

This routine is used in the example code found in DCOcalibrate.c. Try it out (with the crystal soldered on, of course), and see what values are obtained for DCOCTL and BCSCTL1 for each frequency it calibrates. If you happen to have an oscilloscope, measure them on P1.4 to see if they're right. (The code finishes on the last calibration done; you can modify the code to end with the one you want to measure, or add code to change to the frequency you want to see.) You can write these values down for future reference, but next time we'll look briefly at writing to the flash memory in the MSP430 so you can save it for use later on!

Reader Exercises:  How good is the calibration done in the factory? Modify the code to find the calibration values for 1 MHz. How do they compare to the values stored in CALDCO_1MHZ and CALBC1_1MHZ? It seems many have reported that the 1 MHz calibration from the factory, at least for early batch runs of the value line devices, is closer to 980 kHz.


How much does temperature affect the results? Place your LaunchPad somewhere warm for a while (or cold; a freezer might not be best, though-- too much water around!) and re-run the code. How much difference is there in the calibration values?


Could you use a calibrated frequency to go the other direction and measure the crystal frequency? Imagine doing an experiment to see how the four XCAPx settings might affect the crystal. Which ones oscillate? How much does the frequency change if you use 10 pF instead of 12.5 pF? See if you can write some code to find out!

Tutorial 16b: UART Definition

Universal
So what exactly makes the Universal Asynchronous Receiver-Transmitter universal? The UART has a long history, starting way back in the 1840's with some of the first telegraph systems. Back then, when the telegraph key was held down, a current would flow in the receiver, pushing a stylus into a strip of paper, leaving a "mark". The Morse code signals sent would then visually display on the paper, making it simple to read the transmitted message. Of course, it didn't take long before the operators got so used to hearing the patterns of clicks that they found they could just as easily listen to the message as write it on a piece of paper, and sounds began being used instead of a mechanical system. Of course, the sounds would turn on when a current was flowing in the receiver, so the signal was still divided into "marks", where current was flowing, and "spaces", where it was not. In other words, the "standard" had changed from a stylus on a paper to listening by ear, but the "protocol" of Morse code stayed the same.

Morse code was a phenomenal technology change, making it possible to send messages easily over very long distances, particularly when radio was implemented, and wires connecting between the source and destination were no longer needed. While that was happening, someone realized a financial benefit could be obtained by using the technology, and hence was born the first ticker tape machine for the stock market. These machines changed the technique slightly. Instead of using special codes for each character, a series of pulses would be sent to turn a printing wheel from its current position to the next letter to be printed. A special pulse signal would instruct the printer to stamp the current letter onto the tape. As technology improved, rather than a rotary printing wheel the Baudot code was developed as a new protocol, equating particular pulse patterns to particular characters.

Like telegraphy, the teletype grew with the new technologies of radio and, in particular, the computer. Teletype machines became useful not only as a means of communication between people, but also as an interface to early computers. Instructions could be sent by typing a particular pattern of keys, sending a particular pattern of pulses to the computer. Results would be sent back with a similar pattern of pulses to a printer, which would translate them back to the letters and numbers we needed to understand them. But even though the technologies had taken different paths, both came from the same beginnings with Samuel Morse. As such, some characteristics and naming conventions stuck; in particular the use of "mark" and "space" to designate when current was flowing (logic high) and when it was not (logic low).

In computers, a change was made from detecting current flow to just measuring a voltage. Some of the conventions continued, which is why in the RS-232 standard has logic high as a negative voltage. The negative voltage originally would open the current of a teletype machine to produce a "mark" signal. A positive voltage would cut off the current, producing a space. As it turns out, different circumstances (and sometimes just different companies) would require a slightly different standard for sending serial data. connectors and voltage levels for mark and space wouldn't be the same, but the protocol (the way of encoding the characters in pulses) used would carry over. In particular, the use of transistors made it easy to create a universal system that could be understood by any computer or device, as long as each device had something to convert the transistor logic (TTL) signals into whatever standard they expected. The protocol was changed as well, using ASCII to encode the data into digital information. Thus was born the Universal Asynchronous Receiver-Transmitter. (Note that TTL uses positive voltage, be it 5 V, 3.3 V, or anything else, for "mark" and 0 V for "space".)

Asynchronous
Now that it's clear what makes the UART universal, let's look at what is meant by asynchronous. In radio, you can send a message (by voice, digital code, morse code, or whatever), but the message cannot be received unless someone is listening. For serial communication, it requires more than just a signal saying data is ready to be transmitted, unfortunately. Imagine a system where I'm going to send a message to you by holding up a giant sign. We agree before hand that at 1:32 PM I will put the sign up, and at the designated time you look in my direction, see the sign, and read the message. This would constitute a parallel type of transmission--each letter was visible all at once. Now let's say I just don't have access to a big enough piece of paper to write the whole message, but I can send you one letter at a time. So we agree that every 10 seconds, I'll hold up a new letter. You come at the specified time and see me hold up the first letter, which you record on a piece of paper. Every 10 seconds you look back, and I'm holding a new letter up and you record it. This is serial communication. But what happens if one of us has a bad clock, and it's saying 10 seconds are up when, say, 12 seconds have passed. Eventually the mismatched timing causes you to either record the same letter twice or miss a letter completely, depending on whose clock is faster. In order to ensure the message gets through, our clocks need to be synchronized.

There are synchronous methods of serial communication, including both SPI and I2C, which we'll address in the future. These methods have synchronized clocks by using the same clock for the sender and the receiver. The disadvantage is that sharing a clock means another wire. It's clear from the history that developed the UART why a clock signal was not included along with the message; instead, both the transmitter and receiver agree before hand at what rate the data will be sent. This allows sender and receiver to have their own clocks, which don't have to be synchronized in terms of when the second hand ticks, but it does require that each person's clock is accurate. Asynchronous communication simplifies the connection by not needing a second signal in parallel with the data, at the cost of needing an accurate way to time intervals between data.

Receiver-Transmitter
Enough history; let's look at how the UART actually transmits information. Whatever protocol we may be using, we are able to encode data as a series of 1's and 0's. We can encode a number as its binary representation, or we can encode a character as a particular binary number. In any case, we have a certain number of 1's and 0's to send. In a UART, we also add on at least two extra bits: one to designate the start of a new set of data, and one to designate the end. These start and stop bits with the data bits in between constitute what we call one "frame" of data. Using ASCII encoding, often 7 bits of data are sent. In addition, a 10th bit would be sent between the data and the stop to help determine if the data received was correct or not. If the sender and receiver agree that every frame will have an even number of 1's in it, then this "parity bit" would be 1 or 0, depending on the number of 1's in the rest of the message. The receiver could then look at the 7 data bits and parity bit, add up the number of 1's, and if the total number is even be confident that they received the right message. In 8 and 16 bit systems like a microcontroller, it could also make sense to send data in 8 bit segments instead. Often times no parity bit is included in this case, to keep the total data length of each frame to 10 bits. The compromise is that there is no way to check for errors in the transmission, but generally error checking is only necessary under particular circumstances.

Let's say we want to encode the letter "D" using 7-bit ASCII encoding and odd parity. The ASCII code for the letter "D" is 0x44, or 0b1000100 in 7-bit binary. Now we face a choice: do we send the least significant bit first, or the most significant bit first? The typical protocol used in UART is what we call "little-endian", meaning we start with the least significant bit. (This makes sense when you think in terms of a shift-register; the SR in a UART pushes bits from high to low, so you send the lowest bit first.)

Representation of the ASCII character "D" in UART TTL.

UART uses logic high as the default (or idle) state. So to start a message, we want to change from high to low. Thus, our start bit will be a 0. Likewise, to stop we want to go back to the default state, so the stop bit is 1. So far, our total encoding is now "00010001x1", where x represents our parity bit. We want odd parity, and there are two 1's in the representation for "D", so we set this bit to 1 to ensure an odd number in the whole message. Our final message is the 10 bit stream "0001000111". (If we do this with 8-bit data and no parity, we would have "0001000101", this time the 2nd to last bit being the most significant bit in the 8-bit code 0b01000100.)

Hopefully this gives you a clear picture on how UART works. There are really no limitations on the protocol you use, so long as you have a start and stop bit. As long as the sender and receiver agree on what goes in the middle and at what rate the information comes, it will work. The standard protocols such as those illustrated here are convenient as it's very simple to use a computer to read the data coming from the microcontroller. In addition, keep in mind that there are standard speeds for transmitting bits (bits per second (bps) or baud), which are leftover from the old teletype days. In any case, many systems are limited to using these rates, so it's often a good idea to standardize to them. If you have your own internal system, use what ever baud rate is convenient, but do remember that a lot of computers and devices may expect one of the more conventional rates, like 300, 1200, 4800, 9600, or 115200 baud.

We've identified one of the key things we'll need for a successful UART: a good clock. Next time we'll look at some options, their limitations, and how to implement them.

Reader Exercises: Using 7 bit encoding with even parity, what would the bit stream look like to send the character "j"?  How about the character "k"?
Using 8 bit encoding with no parity, what would the bit stream look like to send the newline character, "\n"?  How about the character "&"?

Tutorial 16a: Getting Serial

In order to do an actual scientific experiment using the MSP430, we need one more tool. To be fair, we could do with what we've covered so far, but it requires constant (or at least regular periodic) monitoring of the equipment, and manual recording of the data displayed on the LCM. No, what we need is a way to automatically record the data when it is taken.

There are two different paths open to us at this point: the MSP430 has on-board flash memory. We could use it to record multiple measurements. The other, more complicated path is to learn how to communicate between the LaunchPad and a computer via USB. There's some elegance in starting with the former, as our focus to this point has been on the LaunchPad itself, but unfortunately we'd need a way to transfer the data from the flash memory to a useable location anyway, which more or less requires connection to a computer. So even though it will delay getting to some of the cooler things we can do with the MSP430, it's time we tackle serial communication. Once we have this piece mastered, we'll start a little science experiment that will take me a few days/weeks to complete. During that time, we'll begin looking at recording to flash, communicating with external peripherals, and how to put all the pieces together for remote data collection. We'll also start looking at alternative power systems, system control, and other great things that will completely open the field of what's possible with a microcontroller. The future looks bright; but first we'll have to tackle this difficult task.

Well, things aren't really so bleak... serial communication isn't that complicated. In fact, most MSP430 devices have peripherals built in already for that very purpose, making it simple to do. However, of the two devices that come with the LaunchPad, only the G2231 has one of these peripherals, and it only has two modes of operation, conspicuously missing the one we really need first: the Universal Asynchronous Reciever/Transmitter, or UART. So, instead, we are going to turn to learning to implement this functionality in software.

Fortunately, there's some real advantage to this; a solid understanding of how serial communication works helps us understand how to process and record scientific data. In fact, when we get to the USI/USCI peripherals, looking at other modes of communication such as SPI and I2C, we'll take the time to understand how these methods send data. (The particular implementation of a serial communication system is called a protocol. There are even more protocols available, including Bluetooth, Wi-Fi, and ZigBee, which are cool things we'll tackle some day!)

You might be asking, "Why are we going to rehash software UART? Lots of people have published articles about it already, and lots of code and examples are available." Well, I'd respond that there are two reasons. The more philosophical reason is that you become a better scientist when you understand how the tools you're using work; Einstein once said you don't really understand anything until you can explain it to your Grandmother. The more practical reason is that none of the articles I've perused give much explanation to why the code is set up the way it is. That's our goal here: by completely dissecting the software UART, we learn how serial communication works, and get a thorough example of using the MSP430 peripherals to our advantage in getting jobs done. We'll also do a very thorough job, starting with just transmission (I guess technically it would be UAT), then moving to just reception (likewise UAR), then designing a full-on UART transceiver. Along the way, we'll talk a bit about crystals as well as learn about calibrating our DCO. We'll even talk about saving DCO calibration to the flash memory, and introduce the concept of a checksum. (So we'll see a little bit about writing to flash memory soon after all!)

If that sounds like a lot to cover, it is. I'll do my best to keep the posts coming regularly and quickly, so that we can move on to more advanced ideas soon. There is motivation for approaching this topic in this way at this time, however. These tutorials have always been designed as notes from my own learning. As a result, sometimes the methods/styles have been a little disjointed, but one of the goals of this blog was to put together a curriculum that could be used to teach science students in a one-semester course on microcontrollers. (After graduation, I'll gather, edit, and format these tutorials into a book that can be downloaded for just such a purpose.) I think the material we've covered to this point fits about a one semester course very well, so think of this tutorial as the final project for the course. It's a bigger concept that will take a while, but will draw on our knowledge from the other peripherals and skills we've learned. The fact that we'll introduce some new ideas along the way will add to the sum total of knowledge taken away from this course. So strap in; we're going to start the final for MSP430 101!

Question for Readers

We're fast approaching the end of what I would call the 'basic tutorials', and the point where I'll move on to interfacing with the real world and other cool toys and devices. Unfortunately, that means I need some cool toys and devices. In brainstorming ways to help fund this little hobby, a friend suggested to me that I might consider using Google AdSense on this blog. If I were to do so, I would want it to be unobtrusive, as my intent is not to sell things for other people. How would you feel if I were to do this? Would I be better off thinking of another way to raise a little hobby revenue?

Tutorial 15b: Using ADC10

Like the Comparator_A+ peripheral, ADC10 has a wide range of operating modes and features. It can also integrate with other peripherals (such as Timer_A, of course), making it a very powerful tool in scientific measurements. Today we'll look at basic configuration of ADC10 in preparation to do a full scientific experiment using the MSP430. Keep in mind that this tutorial requires a device with the ADC10 peripheral, such as the G2231. The G2211 chip that comes with the LaunchPad will not work in this tutorial.

First, let's examine some of the features of the ADC10 peripheral. ADC10 of course requires a clock, and can source from any of the three clocks in the MSP430 (and subsequently from a crystal, DCO, or VLO). In addition, ADC10 comes with its own internal clock that can be used independently of the system clocks. This clock is typically in the 5 MHz range, but is uncalibrated and thus varies from chip to chip, as well as with operating voltage and temperature. A major advantage to the internal oscillator is that it can remain in operation even when other clocks are powered down in an LPM.

ADC 10 can connect to up to 16 different inputs. Typically, 8 of these are external inputs, 4 are internal inputs, and 4 are other references (on some devices, they are extra external inputs for a total of 12). The G2231 device has 8 external inputs (on each of the pins in P1), and also has a temperature sensor built into the chip as an internal input (in addition to the other three internal inputs, which have to do with voltage reference comparisons).

Like the Comparator_A+, ADC10 requires a reference voltage for operation. In fact, it can operate with two reference voltages for the upper and lower bounds of conversion. The upper reference can be anywhere between 1.4 V and Vcc (up to 3.6 V). The lower reference can be between 0 and 1.2 V. There are two references available inside ADC10 at 1.5 V and 2.5 V, and it can also use Vcc in addition to an external reference.

Finally, the ADC10 also has 4 operating modes. Two of these modes sample only a single channel. The other two modes cycle through a specified set of the 16 possible inputs. Each single/sequence type can be done only once, or repeated. (Note: in a sequence mode, you must use the inputs in order. If you want to sample 3 inputs, you must use A0, A1, and A2. Unfortunately, the only way to sample arbitrary inputs is to use single channel mode and change channels in software.)

That covers the bulk of the dizzying ways to configure the ADC10; it's a lot to sort through, so keep in mind that we're only going over it to have the different things you can do in the back of your mind. The best way to learn how to use all of the features is by example, so let's look at a simple example by modifying the capacitance meter project to a voltage meter. This meter will be restrictively useful, as it will only be able to measure voltages between 0 and 3.3 V, but it will illustrate the idea. We'll display the output on the LCD as before, but the code can easily be modified to pause in the debugger to find the result as was done at first with Comp_A+ if you don't have an LCD. For the LCD display, we'll use the single-channel mode and repeat the measurement in software, which makes it easier to use the debugger to see the result.

The x2xx User's Guide gives a set of diagrams to explain the process used in each of the four modes. For example, in the repeat single-channel mode, the peripheral is turned on and enabled. The ADC10 is triggered to start a conversion, which is stored upon completion. If interrupts are being used, the flag is set, and the ADC10 returns to on of three steps, depending on just how we set it up. Keep in mind this all happens within the ADC10 module itself, leaving the MSP430 free to perform any other actions it needs to. You can use the ADC10 interrupts to do something with the code after samples are taken.

Our code will instead use single-channel mode, which is very similar to the repeat single-channel mode, but without the repeat part. =) I've chosen this mode because I won't be using a low power mode, and it's easier to coordinate timing so that a sample isn't taken and finished while waiting for the LCD to update. While a new conversion is occurring, the code will trap in a loop before writing the measured sample to the LCD for display. Once that's finished, a new conversion will be started to update the measurement.

There are 8 registers associated with ADC10; of these, 4 are used to configure the peripheral. One is used to store the individual samples, and three are used to control transferring the sample data for storage. (More on this later; for now we're going to keep it simple and only worry about 5 registers!)

While this sounds like a lot of configuration, fortunately two of the registers are used solely for configuring the inputs. ADC10AE0 enables the ADC function of the external pins being used. (This is necessary, because P1SEL changes the operation of those pins to a function other than ADC; since it's a binary value, P1SEL can only configure two different operations. This register frees up those pins for uses other than just ADC!) ADC10AE1 performs a similar function, but only for devices with more than 8 analog inputs.

We'll look at the other two configuration registers in more detail. ADC10CTL0 handles some of the base configurations of the peripheral-- the voltage references, sampling time and rate, and handling power and interrupts for the ADC. ADC10CTL1 controls the inputs, clock, mode, and data formatting. Here are the essential pieces for each register (we won't cover all of them today):
ADC10CTL0

• SREFx (Bits 15-13): These select one of 8 different configurations for the upper and lower references for the ADC.
• SHTx (Bits 12-11): These select 4 different sampling times for the ADC. The voltage is held constant during conversion by charging a capacitor; these control the amount of time you allow for charging. Obviously more time ensures a more accurate sample, but limits the sampling rate achievable by the device and risks having the voltage being measured change during the sampling time. You can select 4, 8, 16, or 64 clock cycles (of ADC10CLK).
• REF2_5V, REFON (Bits 6,5): Selects between 1.5 V and 2.5 V references and turns the reference on/off.
• ADC10ON, ENC,ADC10SC (Bits 4,1,0): Turns on the ADC, Enables Conversion, and Starts Conversion respectively.
• ADC10IE, ADC10IFG (Bits 3,2): Interrupt enable and flag.

ADC10CTL1

• INCHx (Bits 15-12): In single channel mode, selects the channel to sample. In sequence mode, selects the highest channel to sample.
• ADC10DF (Bit 9): change between straight binary data and 2's complement data.
• SSELx, DIVx (Bits 4-3,7-5): chose the clock source and divide the clock frequency by 1-8.
• CONSEQx (Bits 2-1): Select the sequence mode.
• BUSY (Bit 0): a read-only flag that indicates when the ADC is in the middle of a sample/conversion cycle.

 That's a very brief overview; we can't cover all of the features in detail in a reasonable introductory tutorial, so we'll examine more advanced features in the future as they come up. In the mean time, read the User's Guide and documentation to understand more of what all of these do.

Last of all, we'll mention the ADC10MEM register. When conversion takes place, the value is stored and read from here. If ADC10DF is cleared (value 0), we can read this straight away: 0x00 is equivalent to the lower reference, 0x3FF is equivalent to the upper reference, and the intermediate values are a line between the two points. If ADC10DF is set (value 1), the value is stored in 2's complement. This can be useful for transferring data in some configurations, but we'll not need it today.

That does it for a brief (but long!) summary of the basics. Let's look at the simple volt meter now. The code for this project can be seen in VMeterG2231.c. It will require the simple_LCM library from the previous tutorial. There's very little that's new here, and the code should be clear by itself. It uses input A1 on P1.1. You can test the code by using a potentiometer connected between Vcc and ground and connecting the wiper to P1.1. When you turn the potentiometer, you should see the corresponding value change on the LCD.

So much for the basics; next time we'll look at how to store the data for later analysis and start working on an actual experiment.

Reader Exercise: How can you use this same code to measure a larger voltage range? Hint: a simple way to do it uses only two passive components. A trickier task is to be able to measure positive and negative voltages; can you think of a way to do this even though the MSP430 can't use a negative voltage reference? Hint: an op amp might help.